Impregnating varnish for electric motors wound with wire enamelled with polyester-imide



Int. Cl. cos 41/00, 51/36 U.S. Cl. 26031.2 6 Claims ABSTRACT OF THE DISCLOSURE Varnish useful for impregnating electrical structures such as coils has alternate trimellitic diimide and hydroxylated terephthalate polyester groups, is compatible with terephthalate polyester and trimellitic polyimide group containing polyester-imide wire enamel and is soluble in readily available solvents such as butyl acetate, butyl acetate-xylene mixtures and the like.

The present invention relates to an impregnating varnish intended to be used for motors wound with Wire enamelled with polyester-imides. As is known, best results are obtained in impregnation when the resin, of which the solution in a solvent medium constitutes the varnish, has a chemical constitution very close to that used for enamelling the conductors, which assures complete compatibility betwen the two resins forming the final insulation, insures that the thermal class of the enamel is maintained, and gives a homogeneous insulation. In addition, and in order to fulfill its role of bonding between the component parts of the motor and to fill its function as a protective coating agent, it is required that this impregnating resin have good flexibility. It is advantageous that its noncorrosive solvent be easily and rapidly driven oil by a baking cycle of relatively short duration.

A polyester-imide enamelling resin for electrical conductors is described in French patent application 2,637 filed on July 23, 1965 by this company for Mixed Polyester and Tri-mellitic Polyimide Resins Especially Applicable for the Enamelling of Electrical Conductors. This relates to a polymer formed from alternate units of terephthalate polyester comprising two molecules of the terephthalate component for 2.5 molecules of glycol and 0.5 molecule of glycerine, and trimellitic diimide components obtained by condensation of two molecules of trimellitic anhydride with one molecule of methylene dianiline. Another French patent application, 2,660. was filed Dec. 30, 1965 by this company for Polymeric Resins Useful for Insulating Applications and More Particularly for the Enamelling of Electrical Conductors and proposes the use, in place of an aromatic diamine such as methylene dianiline, of the corresponding diisocyanate.

Starting from the investigations made in the course of these studies, new research has led to a composition consisting of a trimellitic polyimide/terephthalate polyester resin, which is flexible and resilient, soluble in butyl acetate alone, or mixed with xylene, and as such suitable for use as a varnish in this solvent mixture for impregnating coils made with polyester-imide enamelled wire.

The present invention has for a primary object impregnating varnishes composed of alternating units of trimellitic diimide, obtained by condensation of trimellitic anhydride and an aromatic diisocyanate, and of hydroxylated terephthalate polyester units, characterized in that the hydroxylated terephthalate polyester results from the transesterification of a mixture of dimethyl terephthalate States Patent and adipic acid with a mixture of neopentyl glycol and trimethylolopropane. Another characteristic is that the combination of the trimelliitc diimide and the terephthalate polyester is caried out by esterification in butyl acetate.

In comparing the chemical structures of neopentyl glycol and ethylene glycol and of trimethylolpropane and glycerine it is evident that the bulkier character of the two first materials in each case results, in the polyester state, in a less compact structure, thus facilitating slippage of the molecules on deformation. Flexibility is equaly known to be obtainable by incorporating a flexible methylene chain such as the radical (CH contained in adipic acid of which the useful proportion is limited by the lowering of thermal stability which this additive brings about. The substitution of trimethylolopropane improves the solubility characteristics considerably by comparison with those imparted by glycerine, the solubility in the butyl acetate-xylene mixture being equally improved by the presence of neopentyl glycol in place of ethylene glycol.

Thus, the choice of polyols for esterifying the terephthalic unit and the inclusion of an amount of adipic acid meet the properties dictated by the double requirement of flexibility and solubility while retaining the desired thermal stability. It has been found that the most advantageous polyester group, taking into account all of the considerations, is represented by the formula Moles Dimethyl terephthalate 3 Adipic acid 2 Neopentyl glycol 4.5 Trimethylolpropane 1.5

corresponding to a structure of five diacid elements for six polyol elements, and designated hereafter by the name polyester 6/ 5. It is better in the form of the polyesterimide combination than the polyester of shorter chain length and lesser adipic content:

Moles Dimethylterephthalate 2 Adipic acid l Neopentyl glycol 3 Trimethylolpropane l designated as polyester 4/ 3.

In all cases it was found advisable to retain a mole ratio between diol and triol of 3:1 with a mole ratio between polyols and diacids between 6/5 and 4/ 3, and a proportion of adipic acid not exceeding 40 mole percent of the total acid groups. The resulting polyesters of short chain and terminated on their extremities by hydroxyl groups contain in addition, on the average, from one to two free lateral hydroxyl groups situated along the chain, the presence of which improves the properties of adhesion and permits the curing of the varnish during baking after impregnation.

With regard to the trimellitic diimide groups, one prepares them in the form of molecular units with reactive carboxyl groups at their extremities by treating trimellitic anhydride with an aromatic diisocyanate, such as that of methylene dianiline, in a mole ratio of 2:1 in a medium of butyl acetate serving as the final solvent for the varnish.

4 In this reaction it is the anhydride groups of trimellitic tainable under the trade designation Desmodur CT) anhydride which react first to give the structure blocked by trimethylolpropane added in the form of a o II II /C\ /O coon -o0on o 0 II II 0 o and splitting off carbon dioxide gas. solution of about 70% in butyl acetate which one incor- If one designates the trimellitic element by T and the porates into the varnish by vigorous mixing. A propordiisocyanate element by D one can represent the central tion of Desmodur CT of 4-6% by Weight of the resin part by the schematic TDT. gives the best results in terms of thermal stability, meas- The reason for the use of the diisocyanate of methylene ured by weight loss in a ventilated oven at 200 C. For dianiline in preference to methylene dianiline itself which a varnish of 4% Desmodur CT this loss rises to 10% in leads to the same reaction by an addition mechanism the course of the first fifty hours but does not exceed resides in the fact that the commercial diisocyanates cOn- 11% at the end of the subsequent 150 hours. The flexitaining a proportion of polyisocyanates are less expensive bility of the varnish obtained can be measured by enameland usable provided certain precautions indicated below ling an aluminum sheet of 50 microns with a layer of are followed. varnish of micron on both sides. After aging the The reaction between the two precondensates is carried enamelled sheet is capable of being bent to 180 degrees out adding the polyester to a non-crystalline TDT gel in without rupture of the interior and exterior films of the butyl acetate solvent. This gel reacts progressively to give varni h, a macromolecular structure shown schemaically as 29 The resin of the present varnish possesses complete compatibility with a polyester-imide enamel used on polyester/T D T/polyeSter/T D T enamelled wires for coils of electric motors and there is y reaction between the OH groups of the Polyester and no attack on the wires by the solvent of the impregnating the carboXylgrOhps Ofthe varnish. This solvent, light and noncorrosive, is easily To Obtain the 6/ 5 Polyester one Introduces Into a flask 30 eliminated thus resulting in short cure temperatures. After fitted with a Vireux column the equivalent weights of impregnation and curing the polyimide impregnating resin shows the physical, mechanical, and thermal char- 3 moles dimethylterephthalate 5 2 acteristics, in particular of flexibility and resistance to 2 moles adipic acid 292 thermal degradation, which ustifies its use for the type 4.5- moles neopentyl glycol 468 of msulauoh Intended- 15 moles trimethylolpropane 201 What I claim as new and desire to secure by Letters Patent of the United States is: adds at thb start about of xylehb whlbh facll' 1. A varnish having alternate trimellitic diimide and itates the operation by solublizing the sublimed dimethyl- 4O hydroxyleted tterephthalate polyester groups said trimeh terephthalate and in assisting removal of the volatile conhhe diimide groups being obtained by eohdehsatioh of Sthubhts which have h by elevation b h two moles of trimellitic anhydride and one mole of arotemperatures, the reaction mixture has become liquid, math: diisocyanate said polyester groups being derived one adds about 1.4 grams of litharge catalyst for the transfrom the trahsesterifieahon of a mixture of dimethyl esterification. Heating is continued, the temperature be ng tterephthalate and adipie acid with a mixture of ehopehtyl raised progressively to about 230 C. until all distillation glycol and trimethylol Propane sueh that the mole ratio of methanol and Watch has ceased The yleld, of h Poly of diol to triol is 3/1 and the mole ratio of polyol to acid ester prepolymer 5 IS 97% 0f the theorehcal yleldranges from about 6/5 to 4/3, said varnish being made In order to obtain the trimellitic diimide, one first disby reacting about 15 parts of the trimellitic ahhydride solves in 2500 grams of commercial butyl acetate the about 1 part of the diisocyanate and about 51 parts E Weight equivalent to 2 gram molecules of trimelliti h the polyester to form a resin, and dissolving the resin a hydndby about 384 grams- ,Wheh b b by Whmhhg solvent selected from the group consisting of butyl acetate complete one adds the equivalent in weight of one gram and a mixture of butyl acetate and Xylene molecule of the diisocyanate of methylene dianiline, about A varnish as in Claim 1 wherein the mole ratio of 250 gFams, dissolved in butyl acetate" By hh in about dimethyl terephthalate to adipic acid as at least 3 to 2. 15 minutes to 100 C. one observes a precipitate of gel 55 3. A varnish as in claim 1 wherein the mole ratio of corresponding to the combination of the two reactants. neohehtyl glycol to trimethylol propane is about The diisocyanate of methylene dianiline is obtainable com- A Varnish as in Claim 1 wherein the solvent is butyl mercially under the trade designation Desmodur VL. At acetate this stage one adds 1279 grams of the polyester 6/5 rep- 5' A varnish as in claim 1 wherein the Solvent is a resenting the equivalent Weight of the oH'polyester'oH mixture of up to two volumes of xylene to one volume unit with the addition of about 1500 grams of butyl aceof butyl acetate tate solvent.

The temperature is raised progressively to 200 C. fh h h g s g ib electncal coll Impregnated with the whereas the hydroxylated terephthalate polyester of ethylene glycol and glycerine is insoluble in butyl acetate, the 3 References Cited polyester of the mixed constituents does dissolve and reacts progressively with the TDT gel. One observes a UNITED STATES PATENTS gradual increase of the viscosity, the stabilization of which 2,935,487 5/ 9 F X 6t a1. 26045 .4 corresponds to the end of the reaction. The chestnut 3,314,923 4/ 967 Muller et al. 26078 colored pasty mass dissolves without difficulty in the bu- 7 tyl acetate to give a clear solution and tolerates cutting MORRIS LIEBMAN, Pnmary Exammer of the acetate with xylene up to the limit of two volumes R ZAITLEN Assistant E i of the latter to one of the former.

The varnish concentration thus obtained is about 33% US. Cl, X,R, resin. An adequate curing agent is tritolyl isocyanate (ob- 260-857 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION patent 3,532,661 Dated October 6, 1970 Henri P.Vayson de Pradenne Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Claim 1, line 50, after "resin" (second occurrence) insert in SiGNFzD Am mcs m (SEAL) .Attest:

Edward m. Fletcher, I

mm: 1:. m. Atteshng Offiw Gonmissioner of Patents USCOMM-DC 60376-969 FORM PO-105D (10-69] 0 us, aovelmunn PRINTING OFFICE nu o-an-ssn 

